High-power digital amplifiers used in communications networks may have requirements to provide several peak power levels to adjust for range and to provide low power modes. In some types of networks, reverse channels are available that provide information to adjust the transmitter peak level to match the desired range such that four or more peak power levels are achieved. The range of required power levels required to be provided by a power amplifier may extend down 30 dB or more from the maximum power level.
One approach to establish multiple power levels is to provide an attenuator for each desired level of attenuation. FIG. 4 shows a portion of a power amplifier in which a pair of two-way switches 10, 11 selectively toggle an RF input 12 between a final amplifier stage 14 and a parallel array of attenuators 16. A pair of 3-way switches 18, 20 selectively toggle between a path 22 with little or no attenuation, a path including a first attenuator 24, and a path including a second attenuator 26. By controlling two-way switches 10, 11 and three-way switches 18, 20, a power output 28 can be provided with four levels of attenuation. However, these switches require high-power PIN diodes and drivers, and a large area is needed to place such components in a circuit. Furthermore, each attenuation level requires a separate high-power attenuator, with additional desired attenuation levels requiring more attenuators. Lastly, input signals of varying strengths may compromise the performance of the attenuators, resulting in unpredictable output power levels. For these reasons, the attenuation strategy as depicted in FIG. 4 may create unrealistic power and cooling requirements for portable communications devices.
It is therefore an object of the invention to provide a digital power amplifier usable at multiple power levels while using a minimum of high-power control components.
It is also an object of the invention to provide such a digital power amplifier that is usable with a wide range of strengths of input power signals.
A feature of the invention is a first digital control attenuator coupled with a second digital control attenuator that supplies an inversely proportional attenuation control level in a gain control loop of the digital power amplifier.
An advantage of the invention is the providing of multiple output power levels with a minimum number of high-power components in its design.